This application claims priority to an application entitled xe2x80x9cApparatus and Method for Transmission Diversity Using More Than Two Antennasxe2x80x9d filed in the Korean Industrial Property Office on May 25, 2000 and assigned Ser. No. 2000-29136, to an application entitled xe2x80x9cApparatus and Method for Transmission Diversity Using More Than Two Antennasxe2x80x9d filed in the Korean Industrial Property Office on Aug. 24, 2000 and assigned Serial No. 2000-49259, and to an application entitled xe2x80x9cApparatus and Method for Transmission Diversity Using More Than Two Antennasxe2x80x9d filed in the Korean Industrial Property Office on Aug. 28, 2000 and assigned Ser. No. 2000-47913, the contents of each of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a transmission diversity system, and in particular, to a system where a UTRAN (UMTS Terrestrial Radio Access Network) operates compatibly with a mobile station (MS) supporting a different transmission diversity technique.
2. Description of the Related Art
A third generation mobile communication system has been developed for high-speed data transmission along with the rapid advance of mobile communication technology and the increase of the amount of data being transmitted. W-CDMA (Wideband Code Division Multiple Access), an asynchronous scheme between UTRANs, is standardized as the third generation mobile communication system for Europe, and CDMA-2000, a synchronous scheme between base stations, is standardized as the third generation mobile communication system for North America. In the mobile communication systems, a plurality of MSs communicate through one base station. When data is transmitted at a high rate, the phase of a received signal is distorted due to fading on a radio channel. The fading reduces the amplitude of the received signal by several decibels to tens of decibels. If the distortion is not compensated for in data demodulation, mobile communication quality is deteriorated. Thus, many diversity techniques have been used to overcome fading.
CDMA usually employs a rake receiver for receiving a signal with diversity utilizing the delay spread of a channel. While the reception diversity relying on the delay spread is applied to the rake receiver, the rake receiver does not operate if the delay spread is less than a threshold. Time diversity relying on interleaving and coding is used for a Doppler spread channel. The time diversity, however, is difficult to apply to a slow Doppler spread channel.
Therefore, space diversity applies to a channel with a small spread delay and a slow Doppler spread channel to overcome fading. For the space diversity, at least two transmission/reception antennas are used. Although the strength of a signal transmitted through one antenna is reduced due to fading, a signal transmitted through the other antenna is received. The space diversity is divided into reception diversity using reception antennas and transmission diversity using transmission antennas. Because it is difficult to install a plurality of antennas in an MS for the reception diversity in terms of cost and terminal size, it is recommended to implement the transmission diversity technique in a UTRAN with a plurality of antennas.
The transmission diversity technique is implemented in an algorithm for receiving a downlink signal and obtaining a diversity gain. The algorithm is generally divided into an open loop mode and a closed loop mode. In the open loop mode, if a UTRAN encodes a data signal and transmits the coded signal through diversity antennas, an MS receives the signal from the UTRAN and obtains a diversity gain by decoding it. In the closed loop mode, if the MS estimates channel environments that signals transmitted through transmission antennas of the base station will experience, calculates weights that maximize the power of reception signals for the transmission antennas based on the estimated values, and transmits the weights as signals to the UTRAN on an uplink channel, the UTRAN adjusts the weights of the antennas based on the weight signals received from the MS. To help the MS estimate the channels, the UTRAN transmits pilot signals through the respective transmission antennas to the MS. Then, the MS estimates the channels according to the pilot signals and acquires optimum weights based on the channel information.
Transmission diversity is applied in a feed-back mode in U.S. Pat. No. 5,634,199 entitled xe2x80x9cMethod of Subspace Beamforming Using Adaptive Transmitting Antennas with Feed-Backxe2x80x9d and U.S. Pat. No. 5,471,647 entitled xe2x80x9cMethod for Minimizing Cross-talk in Adaptive Transmission Antennasxe2x80x9d. While the former proposes channel estimation and feed-back in a perturbation algorithm and a gain matrix, this is a blind scheme that is not suitable for a system with pilots due to a slow convergence speed for channel estimation and difficulty in obtaining accurate weights.
The 3GPP (3rd Generation Partnership Project) specification (Release 99) for UMTS (Universal Mobile Telecommunications System) has suggested quantization and feedback of weights for two antennas. It describes only the case in which an MS supports 2-antenna transmission diversity. The specification made no comment on signal transmission from a UTRAN with transmission antennas and signal transmission and reception in the case where a 2-antenna transmission diversity MS coexists with a 4-antenna transmission diversity MS. Expansion to four antennas by adaptively using a conventional method of expanding signal transmission through one antenna to signal transmission through two antennas is not valid for the 2-antenna transmission diversity MS. Simultaneous use of a signal transmission method using two antennas and a signal transmission method using four antennas also has the problem of power imbalance between the antennas.
Different pilot signals can be transmitted through a plurality of antennas by time division multiplexing, frequency division multiplexing, and code division multiplexing. In W-CDMA, code division multiplexing can be performed with the use of multiple scrambling codes, channelization codes, or multiple orthogonal pilot symbol patterns in order to transmit different pilot signals through the antennas.
In general, a high diversity gain and an SNR (Signal to Noise Ratio) gain of up to 3 dB are acquired by using two transmission antennas, as compared to a conventional system using a single transmission antenna. If transmission diversity is implemented with more than two antennas, an additional diversity gain is obtained besides the diversity gain in a two antenna-transmitter and an SNR gain increases in proportion to the number of antennas. The additional diversity gain is less than that obtained from the 2-antenna transmission diversity but since the diversity order increases, the diversity gain is very high if the SNR (Eb/No) increases.
The 3GPP specification (Release 99) describes a UMTS system operated with 2-antenna transmission diversity but considers the need of transmission diversity using more than two antennas. Consideration should also be given to a transmission/reception framework for a mobile telecommunication system where an existing MS receiving signals from two transmission antennas coexists with an MS receiving signals from more than two antennas. That is, even if an MS designed to communicate with a UTRAN with 2-antenna transmission diversity is located within the coverage area of a UTRAN supporting more than 2-antenna transmission diversity, the MS should operate normally, and vice versa for an MS designed to communicate the UTRAN with more than 2-antenna transmission diversity. It is also necessary to ensure compatible operation of the more than 2-antenna transmission diversity UTRAN with the 2-antenna transmission diversity MS.
The need for compatibility is more pressing for a common pilot channel (CPICH) and a common data channel (CDCH). While a dedicated channel transmits a signal adaptively to a given number of antennas according to the characteristics and version of an MS, the common pilot channel and the common data channel must operate in both a lower-version MS operated in the conventional 2-antenna transmission diversity scheme and a higher-version MS operated in a more than 2-antenna transmission diversity scheme. That is, a common channel is transmitted with stronger power than a dedicated channel because the system should give higher signal reliability to the common channel. Therefore, if an antenna transmission diversity gain is obtained from the common channel, communications can be conducted with low transmission power, thereby increasing system capacity. In other words, the number of subscribers allowable for the system can be increased.
A transmission antenna system refers to a system that transmits signals through a plurality of antennas. A transmission RF system including a low noise amplifier (LNA), for example, is effective in terms of cost and efficiency as long as it uniformly distributes the power of signals transmitted through the antennas. Otherwise, antennas are relatively difficult to design and their cost is high. When transmission power balance is set between transmission signals of the antennas through power distribution, only efficient designing of a transmission/reception system ensures compatibility between the different transmission diversity schemes.
An object of the present invention is, therefore, to provide a signal transmission method and apparatus for transmission diversity using four antennas in a UTRAN.
Another object of the present invention is to provide a reception method and apparatus for receiving signals from a 4-antenna transmission diversity UTRAN in an MS.
A further object of the present invention is to provide a signal transmission method and apparatus in a system operated in transmission diversity schemes using different numbers of antennas.
Still another object of the present invention is to provide a pilot signal transmission method and apparatus in a system operated in transmission diversity schemes using different numbers of antennas.
Yet another object of the present invention is to provide a pilot signal reception method and apparatus in a system operated in transmission diversity schemes using different numbers of antennas.
Still further object of the present invention is to provide a pilot signal reception method and apparatus for effectively utilizing limited orthogonal code resources in a system operated in transmission diversity schemes using different numbers of antennas.
The foregoing and other objects are achieved by providing an antenna transmission diversity method and apparatus. According to one aspect of the present invention, in a transmitter of a UTRAN having at least four antennas, a first adder is connected to a first antenna, and adds a first spread signal produced by spreading a first symbol pattern with a first orthogonal code and a second spread signal produced by spreading the first symbol pattern with a second orthogonal code orthogonal to the first orthogonal code. A second adder is connected to a second antenna, and adds the first spread signal and a third spread signal produced by spreading a first inverted symbol pattern resulting from inverting the phase of the first symbol pattern with the second orthogonal code. A third adder is connected to a third antenna, and adds a fourth spread signal produced by spreading a second symbol pattern orthogonal to the first symbol pattern with the first orthogonal code and a fifth spread signal produced by spreading the second symbol pattern with the second orthogonal code. A fourth adder is connected to a fourth antenna, and adds the fourth spread signal and a sixth spread signal produced by spreading a second inverted symbol pattern resulting from inverting the phase of the second symbol pattern with the second orthogonal code.
According to another aspect of the present invention, in a UTRAN transmitter, a first adder is connected to a first antenna, and adds a first spread signal produced by multiplying a first symbol pattern by a gain constant and spreading the product with a first orthogonal code and a second spread signal produced by spreading the first symbol pattern with a second orthogonal code orthogonal to the first orthogonal code. A second adder is connected to a second antenna, and adds the first spread signal and a third spread signal produced by spreading a first inverted symbol pattern resulting from inverting the phase of the first symbol pattern with the second orthogonal code. A third adder is connected to a third antenna, and adds a fourth spread signal produced by multiplying a second symbol pattern by the gain constant and spreading the product with the first orthogonal code and a fifth spread signal produced by spreading the second symbol pattern with the second orthogonal code. A fourth adder is connected to a fourth antenna, and adds the fourth spread signal and a sixth spread signal produced by spreading a second inverted symbol pattern resulting from inverting the phase of the second symbol pattern with the second orthogonal code.
According to a third aspect of the present invention, in a signal transmitting method in a UTRAN, a first spread signal produced by spreading a first symbol pattern with a first orthogonal code is added to a second spread signal produced by spreading the first symbol pattern with a second orthogonal code orthogonal to the first orthogonal code and the sum is transmitted through a first antenna. The first spread signal is added to a third spread signal produced by spreading a first inverted symbol pattern resulting from inverting the phase of the first symbol pattern with the second orthogonal code, and the sum is transmitted through a second antenna. The fourth spread signal produced by spreading a second symbol pattern orthogonal to the first symbol pattern with the first orthogonal code is added to a fifth spread signal produced by spreading the second symbol pattern with the second orthogonal code, and the sum is transmitted through a third antenna. The fourth spread signal is added to a sixth spread signal produced by spreading a second inverted symbol pattern resulting from inverting the phase of the second symbol pattern with the second orthogonal code, and the sum is transmitted through a fourth antenna.
According to a fourth aspect of the present invention, in a signal transmitting method in a UTRAN, a first spread signal produced by multiplying a first symbol pattern by a gain constant and spreading the product with a first orthogonal code is added to a second spread signal produced by spreading the first symbol pattern with a second orthogonal code orthogonal to the first orthogonal code, and the sum is transmitted through a first antenna. The first spread signal is added to a third spread signal produced by spreading a first inverted symbol pattern resulting from inverting the phase of the first symbol pattern with the second orthogonal code, and the sum is transmitted through a second antenna. A fourth spread signal produced by multiplying a second symbol pattern by the gain constant and spreading the product with the first orthogonal code is added to a fifth spread signal produced by spreading the second symbol pattern with the second orthogonal code, and the sum is transmitted through a third antenna. The fourth spread signal is added to a sixth spread signal produced by spreading a second inverted symbol pattern resulting from inverting the phase of the second symbol pattern with the second orthogonal code, and the sum is transmitted through a fourth antenna.
According to a fifth aspect of the present invention, in a transmitter of a UTRAN, a first adder is connected to a first antenna, and adds a first spread signal produced by spreading a first symbol pattern with a first orthogonal code and a second spread signal produced by spreading the first symbol pattern with a second orthogonal code orthogonal to the first orthogonal code. Here, the first orthogonal code has chips of all 0s and the second orthogonal code has 0s in the first half chips and 1s in the latter half chips. A second adder is connected to a second antenna, and adds the first spread signal and a third spread signal produced by spreading a first inverted symbol pattern resulting from inverting the phase of the first symbol pattern with the second orthogonal code. A third adder is connected to a third antenna, and adds a fourth spread signal produced by spreading a second symbol pattern orthogonal to the first symbol pattern with the first orthogonal code and a fifth spread signal produced by spreading the second symbol pattern with the second orthogonal code. A fourth adder is connected to a fourth antenna, and adds the fourth spread signal and a sixth spread signal produced by spreading a second inverted symbol pattern resulting from inverting the phase of the second symbol pattern with the second orthogonal code.